Dietrich



Dec. 22, 1964 B. D|E\TRICH 3,162,802

VOLTAGE DIVIDER Filed Jan. 25, 1965 F a G 2 INVENTOR.

BERNHARD DIETRICH AT OR NEY United States Patent 3,162,802 VOLTAGEDIVEDER Bernhard Dietrich, l rciburg im Breisgau, Germany, as

This invention relates to voltage dividers generally and, in particular,to high-resistance continuously-adjustable potentiometers and likedevices.

It is well known in the art that purely resistive voltage dividers ofhigh impedance exhibit substantial nonlinearity in response at highfrequencies, i.e., in excess of 100 kc. due to the effect of distributedcapacitance In practice, therefore, it is necessary to design voltagedividers intended for high frequency applications with low resistancebecause substantially linear response can be achieved only when the timeconstants (the product of the resistance element and its distributedcapacitance) are much lower than the reciprocal of the operatingfrequency. A more detailed discussion of this problem is undertakenhereinbelow.

One approach to the elimination or suppression of frequency effects inhigh-frequency, high-impedance voltage dividers is to connect capacitorsof fixed values in parallel with the distributed capacitance. This,however, is not a satisfactory solution to the problem because itimpairs the frequency range of the device by increasing the timeconstant. As a result the wave form of the divided voltage would vary inaccordance with the setting of the divider. To avoid the increase intime constant it would, of course, be possible to lower the value of theresistive impedance element but then the result is a low-resistancenetwork.

Another possibility to avoid limiting the frequency range of the voltagedivider is to lump the added shunt capacitors in an adjustable rotarycondenser having its shaft mechanically coupled to a rotarypotentiometer which provides the voltage dividing resistance. Thedisadvantage of this arrangement is that it requires the toleration ofrelatively large values of distributed capacitance because adjustment toachieve substantially complete compensation would entail reduction ofthe resistive impedance to the undesired low values. Furthermore, thissystem would raise the need for extensive shielding.

The fundamental object of the present invention is to overcome ormitigate at least one of the problems of the prior art as outlinedabove.

A more specific object is the provision of a high re sistance voltagedivider having substantially linear response over a wide range of highfrequencies.

Another object is the provision of a voltage divider as characterized inthe immediately foregoing object which does not require an adjustablecapacitor nor extensive shielding.

For the fulfillment of these and further objects, the present inventioncontemplates a voltage-divider network which comprises an adjustableresistance having in parallel therewith voltage-dependent capacitanceelements such as semi-conductor diodes. Means are provided for applyinga D.C. potential to the capacitance elements and for adjusting thepotential conjointly with adjustment of the adjustable resistance insuch a manner as to maintain substantially constant the product of theresistance and capacitance.

Additional objects of the invention, its advantages, scope, and themanner in which it may be practiced will be more fully apparent topersons conversant with the art from the following description of anexemplary embodiment taken in conjunction with the subjoined claims andannexed drawings in which like parts are designated by like referencecharacters throughout the several views and FIGURE 1 is a schematiccircuit diagram of a conventional voltage divider network which will bereferred to for explanatory purposes in the following description; and

FIGURE 2 is a schematic circuit of a voltage divider network embodyingthe present invention.

As previously mentioned high-resistance voltage dividers exhibitnon-linear response at high frequency due to distributed capacitanceeffects. In the prior art type of divider shown in FIGURE 1, thedistributed capacitance represented by broken line capacitors C Cshunting the resistance element R of a voltage divider 10'. Asillustrated, voltage divider 1% takes the form of a rotary potentiometerhaving an adjustable tap 12 which divides resistance R into segments Rand R in accordance with its angular. position 0. For the purposes ofexample and ease of description, the network according to the presentinvention will also be described as embodying rotary potentiometers;however, it will be understood that potentiometers having rectilinearlyslidable taps as well as other voltage-divider components can beemployed.

Continuing with FIGURE 1, a pair of input terminals A, B are connectedto respective ends of resistance R; the output voltage is derivedbetween a common terminal B and a terminal A connected to tap 12.

At low frequencies, the division ratio R /R is a function of the angleof rotation 6 of the adjustment tap 12, the function F is determined bythe physical structure of the potentiometer. if a high frequency A.-C.voltage is applied to terminals A, B, the division ratio becomes equalto inwhich F is an indefinite function due to distributed capacitanceand related to both the frequency, f, of the input voltage and theangular displacement 9 of tap 12.

As previously explained, one approach to the elimination or reduction offrequency dependence involves shunting the distributed capacitance C Cwith capacitors C C FIGURE 1. However, this is undesirable because itincreases the time constants of the network to In accordance withthepresent invention, a pair of voltage-dependent capacitance elements suchas semi-conductor diodes D D are connected in parallel with theresistance element R of potentiometer it) as shown in FIGURE 2,Specifically, diodes D D are arranged in series, with the cathode of theformer connected to the anode of the latter. The cathode "of D isconnected to one end of resistance element R the other end of which iscoupled by a capacitor C to the anode of D The fundamental principle ofthe present invention is the suppression of frequency eifects by varyingthe capacity of diodes D D in accordance with adjustment of R R so thatthe RC products or time constants R C and 12 C (where C and C are thecapacitances of the diodes D and D respectively) remain substantiallyconstant over the entire range of adjustment of potentiometer lid.

To this end diodes D and D are reverse-biased from a suitable source, E,of D.-C. potential applied through a dropping resistor R to the anode ofdiode 1),. For adjustment of the bias potential a voltage divider suchas rotary potentiometer id is provided, having a resistance element Rand an adjustable tap 12. Resistance element R shunts diodes D D and hasone end connected to bias potential source B through R The other 3 endof R is connected through a resistance R; to adjustment tap 12 which inturn is connected to a point 14 between diodes D D The adjustment tap ofvoltage divider is also coupled to point 14 by means of blockingcapacitor C As indicated by broken line 16, the respective tape ofpotentiometers 10 and 1d are mechanically linked for conjointadjustment. In the case of rotary potentiomcters, as shown in theillustrated embodiment, taps 12 and 12 can be mounted on a common shaft.

, As the capacitance of reverse-biased diodes D D varies inversely withthe square root of the biasing potential, voltage'divider 10 should havea quadratic voltage-division characteristic in order to maintain uniformvalues of time constants R C and R C over the range of adjustment ofdivider 19. Of course, in the event that voltage-dependent capacitanceelements other than semi-conductor diodes are used, having anexponential characteristic other than quadratic, voltage-divider libwould need to be adapted accordingly.

In the illustrated embodiment, the quadratic characteristic ofvoltage-divider 1% is approximated by means of fixed resistance R inparallel with R.

Capactitors C and C serve to isolate voltage divider 10 and the A.-C.input terminals A, B from the D.-C. bias source E.

By Way of example, in one reduction to practice, the FIGURE 2 circuithad the following specific values of resistance and capacitance:

R=O.1 megohm R'=l.0 megohm R =0.5 megohm C6,

The circuit described exhibits substantially linear response at 200 kc.for voltage division ratios as high as 3.5: 1; a comparable conventionalcircuit at this frequency is limited to a ratio of 1.2: 1.

While there has been described what at present is believed to be thepreferred embodiment of this invention,

i it Will be obvious to those skilled in the art that various changesand modifications may be made therein without departing from theinvention, and it is aimed, therefore, to cover in the appended claimsall such changes and modifications as fall Within the true sipirit andscope of the invention.

What is claimed and desired to be secured by United States LettersPatent is:

1. A voltage-divider network comprising: a resistance having anadjustable tap; a pair of semi-conductor diodes arranged in series withthe cathode of one connected to the anode of the other, the cathode ofsaid other diode being connected to one end of said resistance; acapacitor coupling the anode of said one diode to the other end of saidresistance; a second capacitor coupling said adjustable tap to a pointof common potential between said diodes; a second resistance having asecond adjustable tap, said second resistance having one end connectedto the anode of said one diode; means electrically connecting saidsecond adjustable tap to the other end of said second resistance and tosaid point of common potential; and means mechanically linking saidadjustable taps for conjoint movement.

2. A voltage divider according to claim 1 including a fixed resistanceconnected between said second adjustable tap and said other end of thesecond resistance.

3. A voltage divider according to claim 2 including input terminal meansfor impressing an A.-C. voltage across said first-mentioned resistance;output terminal means for deriving an A.-C. voltage between theadjustable tap and one end of said first resistance; and means forapplying to said one end of the second resistance, a D.-C. voltage forreverse-biasing said diodes.

References Cited in the file of this patent UNITED STATES PATENTS2,712,625 Blitz July 5, 1955 3,095,533 Keizer June 25, 1963 3,109,995Wargo Nov. 5, 1963 3,110,004 Pope Nov. 5, 1963

1. A VOLTAGE-DIVIDER NETWORK COMPRISING: A RESISTANCE HAVING ANADJUSTABLE TAP; A PAIR OF SEMI-CONDUCTOR DIODES ARRANGED IN SERIES WITHTHE CATHODE OF ONE CONNECTED TO THE ANODE OF THE OTHER, THE CATHODE OFSAID OTHER DIODE BEING CONNECTED TO ONE END OF SAID RESISTANCE; ACAPACITOR COUPLING THE ANODE OF SAID ONE DIODE TO THE OTHER END OF SAIDRESISTANCE; A SECOND CAPACITOR COUPLING SAID ADJUSTABLE TAP TO A POINTOF COMMON POTENTIAL BETWEEN SAID DIODES; A SECOND RESISTANCE HAVING ASECOND ADJUSTABLE TAP, SAID SECOND RESISTANCE HAVING ONE END CONNECTEDTO THE ANODE OF SAID ONE DIODE; MEANS ELECTRICALLY CONNECTING SAIDSECOND ADJUSTABLE TAP TO THE OTHER END OF SAID SECOND RESISTANCE AND TOSAID POINT OF COMMON POTENTIAL; AND MEANS MECHANICALLY LINKING SAIDADJUSTABLE TAPS FOR CONJOINT MOVEMENT.